From the handbook “Speech and Audio Coding for Wireless and Network Applications”, B. S. Atal et al., Kluwer Academic Publishers, 1993, pp. 85–92, a variable speech or adaptive rate coder is known for use in a digital cellular telephone system. From speech signal samples, the speech coder generates speech coder data which are typically contained in 20 ms frames. Periods of silence in the speech signal and background noise are coded at a lower rate so that system capacity is increased. For signaling data to be sent, the system can instruct the speech coder to encode the speech at half rate for one or two frames, allowing the remaining bits which would have been used for full rate speech data instead. Because speech is only coded at half rate for 20–40 ms, speech quality is not significantly degraded, though the amount of signaling data that can be conveyed without causing degrading of speech quality is limited.
The proposed TIA/EIA/IS-707 Standard defines data service options for wideband spread-spectrum systems. At a Rate Set 1, at full rate operation of a speech coder in the system, so-called Primary Traffic is conveyed in bits/frame. At a lower rate of the speech coder, the frame may contain signaling traffic bits or so-called Secondary Traffic bits. The signaling bits are to be considered as in-band data and the Secondary Traffic can consist of user data. Particularly when there is a desire to simultaneously convey speech data and a moderate amount of user data, the speech quality is substantially degraded. This is due to the fact that the speech coder is then forced to operate at a lower rate for a great number of speech frames, i.e., not only for frames containing silent speech periods and/or background noise information but also for speech frames carrying meaningful speech data. Typical applications today in which considerable amounts of data have to be conveyed, either uni-directional or bi-directional, are Internet applications, or the like. In this respect, there is a trend to couple or integrate cellular phones with data devices or personal computers.
Thus, what is needed is a mechanism of simultaneously conveying speech data and substantial amounts of user data without substantially degrading speech quality.